[aprssig] The ideal APRS digi

Stephen H. Smith WA8LMF2 at aol.com
Sun Sep 26 18:52:45 CDT 2004

Robert Bruninga wrote on 9/26/2004, 4:18 PM

 > To clarify what I am proposing for APRS alt-input digis:
 > The ideal APRS digi should consist of:
 > 1) RX/TX on 144.39 connected to good APRS style TNC
 > 2) RX on alt input channel for locals using any RELAY-only TNC
 > 3) External Carrier Detects cross connected between TNC's
 > See the drawing at: http://www.ew.usna.edu/~bruninga/digi.gif

You might note that this setup has EXACTLY the same 
configuration/integration/DCD issues as adding a Mic-E function to a 
voice repeater; i.e. additional RX on a non-144.39 channel while 
providing TX holdoff until 144.39 is clear.

I have wrestled with this issue for some time now since no 
currently-available dual-port TNCs support 1200 baud on BOTH ports.

One thought I have had about an inexpensive way to implement this would 
to use a TNC-X with the serial RX-data-out line connected directly back 
into the serial TX-data-in line. (Since the TNC-X exclusively uses KISS, 
the retransmission would be an exact duplicate of the transmission 
received.  The CD and XCD lines would be cross-connected between the two 
TNCs as in Bob's diagram

Unfortunately, it would also not have the digi's call inserted, or 
WIDEn-N decremented, but it would probably be the cheapest way to go 
(except possibly an old TNC-2 clone with a UI-Digi firmware brain 
transplant).  Presumably the digi's primary on-channel TNC beacons would 
statisfy the legal requirements for ID.

Stephen H. Smith                   wa8lmf (at) aol.com

Home Page:                            http://wa8lmf.com

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